Internal combustion engines are intermittenly operating in that the respective power strokes of the engines from the respective cylinders follow each other sequentially. Continuous rotation is obtained by the inertia flywheel to which the engine is coupled. Speed and torque are essentially continuous output values which are averaged, in time, by the inertia of the flywheel; the energy for supplying the output power is, however, generated in single, intermittently sequentially following combustion processes.
It is desirable to optimize the operation of internal combustion (IC) engines, particularly to reduce fuel consumption and, therefore, it is desirable to sense the combustion processes which occur in the various separate cylinders of a multi-cylinder IC engine separately, so that the single individual combustion processes can be appropriately controlled to obtain optimum overall output.
Controlling the combustion processes requires determination of the condition of combustion within the IC engine. Typical parameters which affect combustion are pressure and temperature. In case of ideal combustion, pressure and temperature within the cylinder can provide data regarding the overall process. Actually, the operation of IC engines is also determined by the chemical processes and reactions which occur, including changes in the composition of the fuel and the applied air, so that actual operation cannot be entirely sensed by merely sensing pressure and temperature; yet, observing pressure and temperature provides substantial, although not entirely complete but usually sufficient information regarding changes in the operating parameters over the cycle of operation in the respective cylinders of a multi-cylinder IC engine.
Determination of the temporal and spatial distribution of the combustion event is desirable to determine the exact condition of what happens during fuel injection, the data for ignition timing, and the like. Upon irregular combustion, which is referred to also as "knocking" of the engine, additional data are desirable.
Irregular combustion may result in knocking of the engine. Such knocking occurs under certain operating conditions. Knocking, as usually understood, is caused by oscillations within the audible frequency band of the compressed fuel-air mixture which is triggered by a shock wave. The heat transmission to the piston walls and cylinder walls of the engine is substantially increased during such oscillations. A thermal overload of the surfaces will result, so that knocking should be avoided. For most efficient operation of the engine, it is desirable to utilize the working range of the engine to the greatest possible extent and, therefore, the engine should be operated just below the "knocking limit". It is necessary to have some means which early and reliably indicate knocking or a tendency to knock, so that the operating parameters of the internal combustion (IC) engine can then be so controlled that the engine will operate just below the knocking limit.
Various types of sensors to determine knocking have been proposed. Mechanical systems which sense the transferred oscillations to the engine, for example using a piezoelectric sensor, are easily made but have the disadvantage that such systems are difficult to operate reliably and free from interference and stray signals, since they also may respond to externally generated jolts and oscillations which arise in the operation of a vehicle to which the engine may be coupled, for example over bad roads or corrugated roads.
It has also been proposed to sense and observe the combustion process by optical means. An optical sensor and background literature are described in the cross-referenced application, assigned to the assignee of the present application, Ser. No. 06/214,481, filed Dec. 9, 1980, MULLER et al, now U.S. Pat. No. 4,393,687. This application discloses a sensor which includes light guide fibers or filaments which are positioned to sense the combustion event optically, the light guides being connected to a photoelectric transducer which, in turn, provides output signals to a tuned or filter circuit which is responsive to or tuned to expected knocking frequency to provide an output signal if shock waves occurring within the combustion chamber are of a frequency which results in engine knocking. Of course, the signals derived from the photoelectric transducer can be processed in any desired manner. The aforementioned patent application further describes integration the light guide with a spark plug of an internal combustion engine; or to place light guides in the cylinder head gasket or seal, for eventual connection to photoelectric transducers.
The various previously proposed systems and arrangements to measure or monitor combustion, and particularly to monitor irregular combustion, were directed to determine the actual course of the combustion in any one combustion chamber. This may, however, lead to erroneous output results since different aging and operating conditions arise in respectively different cylinders of a multi-cylinder engine. The paths taken by inlet air or inlet air-fuel mixture to the respective cylinders are, typically, different for different cylinders and can differentially affect the combustion processes in the respective cylinders, depending for example on the design of the inlet manifold or the like.